Reflections on Radar Observations of Mesoscale Precipitation Sandra Yuter, North Carolina State University David Mechem, University of Kansas 24 Jan 2017

After GATE: convective cells to convective+ stratiform transition in low shear Leary and Houze (1979, JAS)

After PRE-STORM: Mature stage of MCS simplified How did this large system form from a small area of convective cells? Radar echo boundary 0°C level Carefully explain convective and stratiform Stratiform Convective Houze et al. 1989, BAMS

Time scales responsible for instantaneous observed radar field Air flows Precipitation particles increasing time integration Nearly instantaneous response to buoyancy and pressure perturbations wparticles=wair-fallspeed Particles take time to grow Particles persist after growth stops

Distribution mean is not always the mode average average Distribution mean is not always the mode average average

Distilling information on 3D structures Contoured Frequency by Altitude Diagram (CFAD) Yuter and Houze (1995b, MWR)

Time Reflectivity Vertical Velocity Florida: Reflectivity distributions indicate growth as snow particles fall while storm still developing strong updrafts and has no distinct stratiform region Time Kansas stratiform region Yuter and Houze (1995ab, MWR)

Vertical air mass transport wair*air*area Evolving Florida storm Time Vertical air mass transport wair*air*area Kansas stratiform region Yuter and Houze (1995c, MWR)

Particle fountain conceptual model Yuter and Houze (1995c, MWR)

2D simulation using Bryan CM1 model with 250 m grid spacing From Matt Parker, North Carolina State University

2D simulation using Bryan CM1 model with 250 m grid spacing From Matt Parker, North Carolina State University

Sustainability in tropical (low shear) Mesoscale Convective Systems TOGA COARE result: Over periods of sustained convective cell activity, the cells never instantaneously occupied more than a small fraction of the radar domain. As cells weakened they evolved into stratiform precipitation, and the stratiform region grew as each cell finished its active convective phase and was added to the stratiform area. The sustainability of convective cells over time thus determined the overall size of a precipitation area. Yuter and Houze, 1998, QJ

S-band Radar Data from Kwajalein 3 Rainy Seasons of S-band Radar Data from Kwajalein Monthly % total echo area > 20 dBZ Aug-Sept 1999 and 2001 drier than 2001 With ± 2 dB calibration uncertainty KWAJEX Yuter et al. (2005, JAMC)

Typical Kwajalein MCS more irregular than idealized MCS Particle Trajectories Idealized Typical of Kwajalein Holder et al. (2008, MWR)

Limits on area fractions Minimum stratiform area fraction increases with increasing total area Maximum convective area fraction does not ever cover more than ~20% of the 300 km diameter radar domain Increases then decreases with increasing total precip area Holder et al. (2008, MWR)

Useful tools are adaptable Image tool use

Model to Observation Comparisons – Storm Duration Aggregated hail 1 km cloud resolving model with explicit microphysics (ARPS) of Ft. Worth Texas storm for time=0 (Smedsmo et al, 2004) Smedsmo et al. (2004, JAM)

Compare among model sensitivity tests Molthan et al. (2016, MWR)

Part of Houze’s legacy Improved understanding of how mesoscale precipitation systems evolve from a small area of convective cells to large areas of convective cells + stratiform precipitation Adaptable tools for analyzing radar data used for a wide range of problems

Recommendation on top priority observation need for meteorology in general Obtain profiles from the surface to ~2 km [800 mb] every 15 minutes with 10’s of km spacing within wintery mix storms and within inflow air for warm-season storms. Such 3D thermodynamic observations would benefit nowcasting, forecasting, model evaluation, and data assimilation for many types of weather